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The Dry Valley Lakes, Antarctica: a Key to Evolutionary Biomarkers on Europa and Elsewhere

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The Dry Valley Lakes, Antarctica: a Key to Evolutionary Biomarkers on Europa and Elsewhere Instruments, Methods, and Missions for Astrobiology XIV. Edited by Hoover, Richard B.; Davies, Paul C. W.; Levin, Gilbert V.; Rozanov, Alexei Y. Proceedings of the SPIE, Volume 8152, pp. 81520R-81520R-8. DOI: 10.1117/12.898763. The Dry Valley Lakes, Antarctica: A key to evolutionary biomarkers on Europa and elsewhere Julian Chela-Floresa and Joseph Seckbachb a The Abdus Salam International Centre for Theoretical Physics, Strada Costiera 11; 34151 Trieste, Italy; IDEA, Caracas, R.B. Venezuela and b P.O.B. 1132, Efrat, 90435 ABSTRACT Most organisms dwell in what we consider to be “normal” environments, while others, which are called extremophiles, may thrive in harsher conditions. These living organisms are mainly of unicellular (both prokaryotes and, to a lesser extent, there are some eukaryotes) But the extremophiles also include multicellular organisms, including worms, insects and crustaceans. In the present work we survey specific extremophiles in some detail. Astrobiology is concerned with all of these extremophiles, as they may be models for extant life in similar environments elsewhere in the universe. In the more restricted search for life through exploration of the Solar System, the main focus is on the preparation of suites of experiments that may attempt to discover the habitability of planets and their satellites. In this context we ask ourselves: What biosignatures can facilitate life detection, both unicellular and multicellular, in extreme environments? The environments that are within reach of present and future space missions include the Jupiter satellite Europa. The ice- covered lakes of Antarctica’s McMurdo Dry Valleys have long been of interest to astrobiology. These environments harbor unique microbial ecosystems that could orient us how to plan our experiments on Europa. Keywords: astrobiology, Europa, terrestrial analogues, extremophiles, 1. INTRODUCTION Microbial life constitutes a substantial fraction of life on on our planet. They are found in ecosystems organized in microbial mats and sometimes into biofilms (Tewari and Seckbach, 2011). For this reason the description of different aspects of microbial mats are fundamental for our deeper understanding of astrobiology. (We refer the reader to a published review for more details, on which Secs. 1-3 of this paper is based, Chela-Flores, 2011.) Amongst the multiple implications of the study of microbial mats emerges the understanding of the early Earth, before multicellularity evolved. Microbial mats may help us to understand the possibility of life elsewhere in the Solar System. Microbial mats are ubiquitous in extreme environments: at high and low temperatures, in hypersaline bodies of water such as the Dead Sea, in hot springs where they not only survive, but they even thrive, being present even in hydrothermal vents on the ocean floor. Other environments suitable for microbial mats are deserts and, specifically the Dry Valleys of Antarctica in the McMurdo region. Sir Robert Scott discovered the Dry Valleys in 1905 (cf., Table 1 taken from Doran et al., 1994; Wharton et al., 1983; Parker et al., 1982). Some of the most interesting lakes in this region are permanently covered by ice. These extraordinary environments present us with an ideal window to glance at significant events that are relevant for ancient life, and even for paleolimnology that is suggestive of the possible perseverance of life on Mars in an earlier Eden-like epoch. The ice- covered lakes of Antarctica’s McMurdo Dry Valleys have long been of interest to astrobiology (Doran et al., 2010). ____________________________________________ [email protected]; Phone/Fax: 972-2-993-1832 Table 1 Statistics of the Dry Valleys lakes in Antarctica 2 Lake or pond Maximum depth Elevation (meters Lake type (meters) above sea level) Lake Hoare 34 73 Perennial ice cover; liquid water Lake Vanda 69 123 Perennial ice cover; liquid water Lake Joyce 37 1677 Perennial ice cover; liquid water These environments harbor unique microbial ecosystems that could orient us how to plan our experiments on Europa. Lake Joyce is of special interest to NASA, as it is ice covered year-round: Its icy surface is 6 meters deep. Yet, even the few percent of light that penetrates through the ice is enough to support an algal ecosystem in the lake. Many of the structures on the lake bottom look like what we see in the Archean rock record from about 3 Gyr before the present (BP), because its waters harbor carbonate structures known as microbialites. These unique structures are formed with layers of cyanobacteria. The research team is interested in how these organisms are able to grow in the dark, cold waters of Lake Joyce: In these environments the extremophiles that are trapped in microbial mats may also be living under the Taylor Glacier in the Taylor Valley. These microbes probably lived in the ocean at one time, but when the floor of the Dry Valleys rose more than a million years ago, the glacier covered seawater when it advanced and trapped the microorganisms in pockets of water (cf., Table 2). Table 2. Microbial life in the Dry Valleys lakes, Antarctica. Organism Domain Habitat Cyanobacteria Bacteria Lakes Chad, Fryxell and Vanda Leptothrix Bacteria Lakes Fryxell and Hoare Achronema Bacteria Lakes Fryxell and Hoare Clostridium Bacteria Lakes Fryxell and Hoare Chlamydomonas subcaudata Eucarya Lakes Bonney (east lobe) (Phylum Chlorophyta) and Hoare Diatoms Eucarya Lakes Bonney, Chad, Fryxell, (Phylum Bacillariophyta) Hoare and Vanda Bryum (a moss) Eucarya Lake Vanda An intriguing feature—Blood Falls—suggests the presence of microbial mats underneath the Taylor glacier. The name is due to the resemblance with a blood-red color waterfall at the glacier's extreme end. Isotopic measurements of sulfate, water, carbonate, ferrous iron and gene analyses imply that a microbial consortium facilitates a catalytic sulfur cycle analogous to the metabolic events that may sustain life elsewhere in the Solar System (Mikucki et al., 2009). This is especially relevant to the icy satellites of the outer Solar System, including Europa, where the Galileo Mission discovered sulfur patches (1995-2003). These stains on the icy surface of the Jovian satellite are suggestive of chemosynthetic products of metabolism (Chela-Flores, 2010) From the point of view of geology and microbiology, some of the best studied frozen lakes are in the Taylor Valley, (Chad, Fryxell and Hoare. Amongst the microbial mats that are permanently thriving in the frozen lakes there are examples of both prokaryotes and eukaryotes. Besides, some of the most interesting geologic paleoindicators for reconstructing the history of these lakes are stromatolites. In the Dry Valleys these structures consist of various species of cyanobacteria, such as Phormidium frigidum, a prokaryote that forms the matrix of most mat types (Wharton et al, 3 1983). Modern organisms analogous to ancient life are to be found in the Dry Valley lakes. What is most significant is that single-celled eukaryotes are amply represented in this Antarctic biota (cf., Table 3). Table 3. A few examples of eukaryotes present in Antarctica. Organism Domain Habitat Diatom shells Eucarya (Bacillariophyta) Lake Vostok (ice core, at depth of 2375m) Caloneis ventricosa Eucarya (Bacillariophyta) Lakes Chad, Fryxell, Hoare and Vanda Navicula cryptocephala Eucarya (Bacillariophyta) Lakes Bonney, Fryxell, Hoare and Vanda Chlamydomonas subcaudata Eucarya (Chlorophyta) Lakes Bonney and Hoare Tetracystis sp. Eucarya (Chlorophyta) Lakes Fryxell, Hoare and Vanda Yeast Eucarya (Ascomycota) Lake Vostok (ice core) Amongst the related paleoindicators that have been found are diatom frustules, cyst-like structures, most likely of crysophycean origin have also been identified. These intriguing lakes contain various taxa of planktonic and benthic microorganisms. These environments are dominated by lower life forms inviting us to search for biomarkers of an earlier biota since grazing, for instance, is totally absent (Doran et al., 1994). Microbial mats in lake Bonney, Chad, Fryxell, Hoare and Vanda have been thoroughly documented, especially since the 1980s. For instance, in these environments microbial mats are known to include heterotrophic bacteria, eukaryotic algae (mainly diatoms) and fungi (Baublis et al., 1991) besides the above-mentioned cyanobacteria. There are some dinoflagellates Gymnodinium and Glenodinium in Lake Fryxell, where in addition protozoan taxa are associated with the algal mats (Cathey et al., 1981). The existence of these permanently frozen lakes adds an extra bonus to our model of the Europan Ocean. Modern organisms analogous to the early Earth biota are found in the Dry Valley lakes. Single celled eukaryotes are represented. In Tables 2 and 3 we have summarized the names of some of the organisms that inhabit in these lakes (Doran et al., 1994, 2010; Wharton et al., 1983; Parker et al., 1981, 1982; Ellis-Evans and Wynn-Williams, 1996). 2. A SOUTH POLE ANALOG TO EUROPA From the point of view of the possibility of the existence of life on Europa, we should consider a lake called Vostok, which is the largest of about 80 subglacial lakes in Antarctica (Siegert et al., 2005). Its surface is of approximately 14,000 km2 and its volume is 1,800 km3. Indeed this Ontario-sized lake in Eastern Antarctica is also deep, with a maximum depth of 670 m. On the other hand, from the point of view of microbiology, the habitat provided by Lake Vostok presents us an analogue for the Europa environment. The ice above the lake
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